There seem to be four quarks involved, but nobody's sure how they're linked.

Two different accelerators have found evidence for a particle that appears to contain four quarks, according to papers published in Physical Review Letters. Although particles with two and three quarks are common, this would be the first time that something containing four quarks has been spotted. Depending on the precise nature of the interactions among the quarks, this could be a discovery that keeps the theoreticians very busy.

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete. But one family of these fundamental particles—the quarks—combine with gluons to make more complex particles called hadrons and mesons. Hadrons include the proton and neutron, and they are formed by combinations of three quarks. Mesons, which are unstable, are comprised of pairs of quarks.

Having only two quarks would seem to make mesons fairly simple when actually they're anything but. There are three families of quarks, each with a particle and antiparticle, and mesons can consist of any combination of these. They also create some of the more amusing nomenclature in physics, with mesons involving a strange quark being referred to as strangeonium, and those with a bottom quark as bottomonium.

Earlier collider experiments had suggested that the presence of a meson with two bottom quarks might be associated with a heavier particle of unknown properties. So, two different teams, one working at a collider in Japan, the other at one in Beijing, decided to look at whether the same was true with charmonium. (Both colliders are electron-positron colliders, which have many advantages, despite their relatively low energies.)

So, the teams looked at events that included a J/ψ meson, which is a single particle (with two names, since two groups announced its discovery simultaneously) that is composed of a charm quark and a charm antiquark. To do this, they scanned the data sets generated by two different detectors: BES III in Beijing, and Belle at the KEK facility in Japan. The researchers pulled out those events that included a J/ψ and a pair of π particles (another type of meson), with the J/ψ being spotted due to its decay either into an electron and positron, or a muon and antimuon. With those in hand, they searched for indications that the J/ψ was the product of the decay of a heavier particle. (They do this by looking at what's called the "structure of the mass spectrum").

Both teams found something at 3.9GeV, which they're terming Zc(3900) due to its apparent mass of 3900MeV. But its presence is coupled to the appearance of charged π particles, which suggests that the new particle itself is charged. This means that it is probably comprised of four quarks. And, as mentioned above, particles with four quarks have not been previously detected.

The results have a statistical significance well above that required to count for discovery in particle physics, and the fact that there seems to be a similar heavy particle for bottom quarks suggests that this may be a common feature for all the quarks. There's also nothing in particular that rules out four-quark particles but we've gotten pretty deep into the era of particle physics without ever detecting one, so the results are surprising.

Most of the debate, however, seems to focus on how exactly could four quarks combine. One option is that they combine in the same way that two quarks combine with gluons to make mesons and three combine with gluons to form hadrons. The alternative would be what some reports are calling a "meson molecule," where a pair of two-quark mesons are held together by an attractive force. The problem with the latter option, as noted by Nature News, is that the molecule should be less stable than its constituent mesons. But the detectors see no sign of it splitting apart before it decays.

Given the clear method for spotting the Zc(3900) laid out by these papers, it should be easy for anybody to comb through their data and look for similar events, which may shed some more light on the particle's properties. And, in the mean time, it's a safe bet that theorists will be looking carefully at various forms of four-quark particles (molecule and otherwise) to see what sort of predictions they could make about the particle's behavior.

It wouldn't surprise me if more examples turn up in other datasets. The result sets from accelerator collisions are generally huge, and since it typically isn't feasible to store and analyze everything, much gets filtered out in favor of what the scientists are actively looking for. But, this is what every scientist lives for, those moments where you tilt your head, frown a little and go "Hmm?"

Why does it follow that a meson molecule must be less stable than the constituent mesons? Imagine a square configuration (Yes, I just said above this was unlikely. Sue me.) with charms in two opposite corners and anticharms in the other two corners. Now each quark has a redundant binding and possibly higher stability.

Why does it follow that a meson molecule must be less stable than the constituent mesons? Imagine a square configuration (Yes, I just said above this was unlikely. Sue me.) with charms in two opposite corners and anticharms in the other two corners. Now each quark has a redundant binding and possibly higher stability.

Okay, so by "four quarks", we mean two quarks and two antiquarks, right? Because when the article says "There's also nothing in particular that rules out four-quark particles", there kind of is: the color force. This particular manifestation of the strong force requires that the total color of a particle always be "white", which can only be done with either three quarks (one of each color) or an even number of quarks and their anti-quarks.

Unless physics is even more surprising than I thought. Which is possible.

Why does it follow that a meson molecule must be less stable than the constituent mesons? Imagine a square configuration (Yes, I just said above this was unlikely. Sue me.) with charms in two opposite corners and anticharms in the other two corners. Now each quark has a redundant binding and possibly higher stability.

Because a meson is a particle-antiparticle pair, which always annihilate. Hence, instability is an inherent property of mesons. Adding more mesons to a "meson molecule" can't increase stability. It just increases the number of annihilation reactions.

"The substructure of the universe regresses infinitely towards smaller and smaller components. Behind atoms we find electrons, and behind electrons, quarks. Each layer unraveled reveals new secrets, but also new mysteries."-- Academician Prokhor Zakharov, "For I Have Tasted the Fruit"

I think we all need to wait until they have something a little more definitive here. Too many times they jump the gun or the media does, sorry ARS, and it turns out to be some quirky detector problem that they hadn't caught earlier. Take a breath and breathe, then wait. Doesn't hurt any.

Well, that's what publishing is for. They've put their method out there, now others can search through their data to see if the appropriate signals show up or just outright try to replicate on different hardware.

Why does it follow that a meson molecule must be less stable than the constituent mesons? Imagine a square configuration (Yes, I just said above this was unlikely. Sue me.) with charms in two opposite corners and anticharms in the other two corners. Now each quark has a redundant binding and possibly higher stability.

Well if there were 4 quarks all bound to each other then that would be a new composite particle like the mesons and hadrons, only with 4 quarks instead of 2 or 3.

If it's a "meson molecule" then that implies two mesons, with something else holding them together like the residual strong force that binds protons together in a nucleus. That force (and all other forces) are vastly weaker than the force binding quarks and gluons together within a proton, and so you would expect the two mesons to be relatively weakly bound as well.

The "more antiparticles equals more annihilation" argument seems like it would apply to both meson-molecule and 4-quark composite as well. Maybe more so the latter.

This is neat! One of the postdocs in our group used to work at BES-III, indeed on J/psi if I recall some of the presentations he did right after joining correctly. And in about two weeks I'm going over to China to meet a bunch of other people from IHEP for a different project, who although probably not directly involved may know something.

"There are three families of quarks, each with a particle and antiparticle, and mesons can consist of any combination of these.."

and

"..composed of a charm quark and a charm antiquark.."

How come a quark and a antiquark don't annihilate each other?

That's why they are their own anti-particles and also far less stable than similar categories of mesons which do not contain matched quark-antiquark pairs. Consider the lifetimes of the charged vs. uncharged pion:

The unmatched quark-antiquark pair lasts 10^9 times longer on average. As to why the meson molecule is less stable - well matched quark-antiquark mesons are their own anti particles. So not only would that be composed of two unstable mesons that will rapidly decay by internal interaction between each one's constituent quarks but also by the interaction between the mesons. A rough way of thinking of this is that it has an additional mode of decay besides the annihilation interaction between the quarks .

Okay, so by "four quarks", we mean two quarks and two antiquarks, right? Because when the article says "There's also nothing in particular that rules out four-quark particles", there kind of is: the color force. This particular manifestation of the strong force requires that the total color of a particle always be "white", which can only be done with either three quarks (one of each color) or an even number of quarks and their anti-quarks.

Unless physics is even more surprising than I thought. Which is possible.

From repeated statements about 2 quark particles containing both quark and antiquark, it appears that multi-quark particles are assumed to be able to contain members of either or both families unless explicitly stated otherwise. Short form--anti-quarks are quarks.

Okay, so by "four quarks", we mean two quarks and two antiquarks, right? Because when the article says "There's also nothing in particular that rules out four-quark particles", there kind of is: the color force. This particular manifestation of the strong force requires that the total color of a particle always be "white", which can only be done with either three quarks (one of each color) or an even number of quarks and their anti-quarks.

Unless physics is even more surprising than I thought. Which is possible.

From repeated statements about 2 quark particles containing both quark and antiquark, it appears that multi-quark particles are assumed to be able to contain members of either or both families unless explicitly stated otherwise. Short form--anti-quarks are quarks.

Yeah it's just after up, down, charm, strange, top, and bottom it would have been hard to give their anti-particules unique names without them starting to sound weird and confusing like giving them names like "weird" and "confusing." So we keep it simple and call the anti-particle of a top quark "anti-top." They're all quarks but "anti" just indicates a particle with those same characteristics but opposite signs for the conserved quantum numbers (electric charge, flavor numbers, etc.)

The physical arrangement of the quarks isn't the weird thing. The weird thing is that there is no combination of four quarks which could bond together.

Here's a rough description of how it works in the standard model:Each individual quark has a 'colour', either red green or blue, but particles made of quarks must be 'colourless'. One way to combine quarks to get a colourless particle is to have red + green + blue. These three quark combinations are called baryons, and that's what most normal everyday stuff is made of. The other way to get a colourless combination is to combine a quark with an anti-quark of the same colour so that the colours cancel out. For example red + anti-red. These two quark combinations are called mesons. And those are the only ways to get colourless combinations.

One could combine a pair of colours and a pair of the corresponding anti-colours to make a colourless quark thing, but such a combination could always be separated into two colourless mesons anyway - so it's easier just to think of that as two mesons rather than a single four quark blob - and those two mesons would not be held together by the strong force which binds quarks. They'd have to be held together by some other force (eg. the weak force, or the electromagnetic force).

Note: the 'colour' is independent of the 'flavour' of the quark, ie. up/down/strange/charm/top/bottom. There can be red 'up' quarks and blue 'up' quarks, and so on. Also, 'colour' and 'flavour' in this context have nothing whatsoever to do with our human senses. They are just arbitrary words to distinguish between different types of things.

The physical arrangement of the quarks isn't the weird thing. The weird thing is that there is no combination of four quarks which could bond together.

Here's a rough description of how it works in the standard model:Each individual quark has a 'colour', either red green or blue, but particles made of quarks must be 'colourless'. One way to combine quarks to get a colourless particle is to have red + green + blue. These three quark combinations are called baryons, and that's what most normal everyday stuff is made of. The other way to get a colourless combination is to combine a quark with an anti-quark of the same colour so that the colours cancel out. For example red + anti-red. These two quark combinations are called mesons. And those are the only ways to get colourless combinations.

One could combine a pair of colours and a pair of the corresponding anti-colours to make a colourless quark thing, but such a combination could always be separated into two colourless mesons anyway - so it's easier just to think of that as two mesons rather than a single four quark blob - and those two mesons would not be held together by the strong force which binds quarks. They'd have to be held together by some other force (eg. the weak force, or the electromagnetic force).

Note: the 'colour' is independent of the 'flavour' of the quark, ie. up/down/strange/charm/top/bottom. There can be red 'up' quarks and blue 'up' quarks, and so on. Also, 'colour' and 'flavour' in this context have nothing whatsoever to do with our human senses. They are just arbitrary words to distinguish between different types of things.

So... complete conjecture, would it be possible to have a baryon with one anti make up a 4 quark particle? I'm imagining a set of red + green + blue + anti-blue quarks. So the "whiteness" requirement might be mostly satisfied (if this is even possible) by the red + green + blue quark whiteness in conjunction with the blue + anti-blue quark whiteness. Or does whiteness have to be of quarks of the same polarity (I fear I'm using that term wrong here), as in, does a red + anti-green + blue particle satisfy whiteness? Alternatively, my 4 quark example could be a red + anti-green + blue + anti-blue particle.

So... complete conjecture, would it be possible to have a baryon with one anti make up a 4 quark particle?

Now that people have managed to help clarify that I wasn't totally wrong on my understanding of the color force, I can say that you're close but not quite right. Standard hadrons with 3 quarks have one of each color: red, green, and blue. Mesons are a quark and an anti-quark (which is just a quark with the opposite signs for the conserved quantum numbers); because that means you'd have, for example, a red and an anti-red, again we have white.

So, for tetraquarks, we still have to stay white. The only way to do that is two quarks and their associated anti-quarks, meaning you'd have, for example, a red, an anti-red, a green, and an anti-green. What you're getting close to is what is theorized for pentaquarks, with five quarks in them. There, you'd have four quarks and an anti-quark, giving the red-green-blue combination of white and then, for instance, a blue/anti-blue pair which is also white.